Semiconductor transistor

ABSTRACT

The semiconductor transistor according the present invention includes an active layer composed of a GaN-based semiconductor and a gate insulating film formed on the active layer. The gate insulating film has a first insulating film including one or more compounds selected from the group consisting of Al 2 O 3 , HfO 2 , ZrO 2 , La 2 O 3 , and Y 2 O 3  formed on the active layer, and a second insulating film composed of SiO 2  formed on the first insulating film.

TECHNICAL FIELD

The present invention relates to a semiconductor transistor, and more particularly to a semiconductor transistor having an active layer made of a GaN-based semiconductor.

BACKGROUND ART

A gallium nitride-based semiconductor transistor (hereinafter, referred to as a GaN-based semiconductor transistor) has, for example, an active layer composed of a GaN-based semiconductor and a gate insulating film formed on the active layer. As the gate insulating film, for example, SiO₂, Al₂O₃, SiNx, GaOx, or HfO₂ is used.

In the GaN-based semiconductor transistor, it is known that when SiO₂ is used for the gate insulating film, the Si of the SiO₂ diffuses into the GaN. In contrast, Japanese Patent Application Laid-Open (JP-A) No. 2008-277640 describes a technique in which, for a gate insulating film, Al₂O₃ is used alone instead of SiO₂.

The technique disclosed in JP-A No. 2008-277640 can prevent the Si of the SiO₂ from diffusing into the GaN. However, a gate insulating film composed of Al₂O₃ has poorer film characteristics than a gate insulating film composed of SiO₂. The poorer film characteristics mean, for example, that leaked electric current is high, breakdown voltage is low, and the like.

SUMMARY OF INVENTION Technical Problem

The present inventors have found that when SiO₂ is used for a gate insulating film in a GaN-based semiconductor transistor, there occurs a mutual diffusion phenomenon in which not only does the Si of the SiO₂ diffuse into the GaN, but also, conversely, an impurity diffuses from the GaN into the SiO₂. When the mutual diffusion phenomenon occurs, the interface state increases, and the interface characteristics between the GaN-based semiconductor and the gate insulating film deteriorate. As a result of intensive and extensive investigations, the present inventors have found that the mutual diffusion phenomenon is attributable to the oxidization power of a compound that forms a gate insulating film.

The present inventors have also found that it is not enough for the compound that forms the gate insulating film to merely have a wider Eg (band gap) than the GaN that forms the active layer, and that the presence of ΔEc (a conduction band barrier) and ΔEv (a valence band barrier) is necessary.

Meanwhile, a GaN-based semiconductor is one kind of so-called wide band gap semiconductors and is expected to be used in a high temperature environment. Assuming the use in a high temperature environment, examples of the compound that forms the gate insulating film of the GaN-based semiconductor transistor include SiO₂ and Al₂O₃, which have a wider band gap than the GaN-based semiconductor and have good high-temperature characteristics.

Al₂O₃ has a stronger oxidization power than SiO₂. Therefore, formation of a gate insulating film from Al₂O₃ prevents, for example, diffusion of impurity from the GaN-based semiconductor into the Al₂O₃, thereby improving interface characteristics, even in a high temperature environment. However, it is difficult to provide an Al₂O₃ with excellent film quality.

SiO₂ has a wider band gap than Al₂O₃ and thus the breakdown voltage is high and the film quality is good. However, since SiO₂ exhibits lower oxidization power than Al₂O₃, due to impurity diffusion or the like, interface characteristics are not favorable. Therefore, in GaN-based semiconductor transistors, even with the use of SiO₂ or Al₂O₃ for a gate insulating film, it has been difficult to improve both the interface characteristics between the GaN-based semiconductor and the gate insulating film and the film characteristics of the gate insulating film.

JP-A No. 2008-103408 describes a GaN-based semiconductor transistor in which a first gate insulating film composed of SiNx is formed on a GaN-based semiconductor, and a second gate insulating film composed of Al₂O₃ or SiO₂, which have greater breakdown strength than SiNx, is formed on the first gate insulating film.

In JP-A No. 2008-103408, the gate insulating film is formed as a composite film (hybrid film), with the intention of providing favorable interface characteristics between a first gate insulating film in the GaN-based semiconductor and the gate insulating film and obtaining a high breakdown voltage in a second gate insulating film. However, the band gap of SiNx is 5.3 eV, which is not wide enough compared to the band gap (Eg=3.3 ev) of GaN, and ΔEv is extremely small. Therefore, even when SiNx is used for the gate insulating film of the GaN-based semiconductor, it can be said that it is difficult to provide both favorable interface characteristics between the GaN-based semiconductor and the gate insulating film and favorable film characteristics of the gate insulating film.

An object of the present invention is to provide a semiconductor transistor that has both favorable interface characteristics between a GaN-based semiconductor and a gate insulating film and favorable film characteristics of the gate insulating film.

To achieve the object, the present invention provides a semiconductor transistor that includes an active layer composed of a GaN-based semiconductor; and a gate insulating film having a first insulating film including one or more compounds selected from the group consisting of insulating films in which an absolute value of standard enthalpy of formation is larger than 228 kJ/mol, and a second insulating film composed of SiO₂ formed on the first insulating film.

In addition, a semiconductor transistor that includes an active layer composed of a GaN-based semiconductor; and a gate insulating film having a first insulating film including one or more compounds selected from the group consisting of Al₂O₃, HfO₂, ZrO₂, La₂O₃, and Y₂O₃, and a second insulating film composed of SiO₂ formed on the first insulating film, is also provided.

Advantageous Effects of Invention

The semiconductor transistor according to the present invention has both favorable interface characteristics between the GaN-based semiconductor and the gate insulating film and favorable film characteristics of the gate insulating film.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing a configuration of the GaN-based semiconductor transistor according to an embodiment of the invention.

FIG. 2 is an explanatory view exemplifying the oxidization power of respective compounds.

FIG. 3 is an explanatory view exemplifying a band structure of respective compounds.

FIG. 4A is a view showing the process of producing the GaN-based semiconductor transistor according to the embodiment of the present invention.

FIG. 4B is a view showing the process of producing the GaN-based semiconductor transistor according to the embodiment of the present invention.

FIG. 4C is a view showing the process of producing the GaN-based semiconductor transistor according to the embodiment of the present invention.

FIG. 4D is a view showing the process of producing the GaN-based semiconductor transistor according to the embodiment of the present invention.

FIG. 5 is a view schematically showing a film-formation apparatus used in the process of producing the GaN-based semiconductor transistor.

FIG. 6A is a view showing the process of producing the GaN-based semiconductor transistor following on from FIG. 4D, according to the embodiment of the present invention.

FIG. 6B is a view showing the process of producing the GaN-based semiconductor transistor according to the embodiment of the present invention.

FIG. 6C is a view showing the process of producing the GaN-based semiconductor transistor according to the embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an exemplary embodiment of the present invention is described in detail with reference to the drawings. FIG. 1 is a cross-sectional view showing a configuration of a semiconductor transistor according to an embodiment of the present invention. A semiconductor transistor 11 is a GaN-based semiconductor transistor having an active layer composed of a GaN-based semiconductor. The semiconductor transistor 11 has a buffer layer 2 formed on a substrate 1, a p-type active layer (p-GaN) 3 formed on the buffer layer 2 using a group-III nitride semiconductor, and a gate insulating film composed of a first insulating film 6 and a second insulating film 7 formed on the active layer 3.

Furthermore, the semiconductor layer 11 includes a gate electrode 8 g which is provided on the active layer 3 with the gate insulating film therebetween, and an n+ type source region 5 s and an n+ type drain region 5 d, which are provided suitably for the gate electrode 8 g and are in ohmic contact with a source electrode 10 s and a drain electrode 10 d, respectively.

Here, the gate insulating film is described. The gate insulating film is a hybrid film composed of the first insulating film 6 and the second insulating film 7. The first insulating film 6 is formed on the active layer 3 and includes one or more compounds selected from the group consisting of Al₂O₃, HfO₂, ZrO₂, La₂O₃, and Y₂O₃. The second insulating film 7 is formed on the first insulating film 6 and is composed of SiO₂.

The present inventors have found that when SiO₂ is used for a gate insulating film in a GaN-based semiconductor transistor, not only does the Si of the SiO₂ diffuse into the GaN, but also, conversely, a mutual diffusion phenomenon in which an impurity diffuses from the GaN into the SiO₂ occurs, and furthermore, that the mutual diffusion phenomenon is attributable to the oxidization power of the compound that forms the gate insulating film. When the mutual diffusion phenomenon occurs, the interface state increases, whereby the interface characteristics between the GaN-based semiconductor and the gate insulating film deteriorate. Therefore, to suppress the mutual diffusion phenomenon, it is necessary to use a compound having a higher oxidization power than SiO₂ for the gate insulating film.

FIG. 2 shows the oxidization power of respective compounds. The oxidization power is represented by a standard enthalpy of formation per 1 mol of metal-oxygen bonding, which is an absolute value of a vertical axis ΔHfO [KJ/mol] in the drawing. As shown in FIG. 2, the oxidization power of SiO₂ is 228; the oxidization power of ZrO₂ is 275; the oxidization power of Al₂O₃ is 279; the oxidization power of HfO₂ is 286; the oxidization power of La₂O₃ is 299, and the oxidization power of Y₂O₃ is 318. Thus, the oxidization power of Al₂O₃, HfO₂, ZrO₂, La₂O₃, and Y₂O₃ are higher than that of SiO₂.

In addition, the present inventors have also focused on the band structure of a compound, and consequently have found that the compound that forms the gate insulating film of the GaN-based semiconductor transistor has a wider Eg (band gap) than the GaN which forms the active layer 3, and the presence of ΔEc (a conduction band barrier) and ΔEv (a valence band barrier) is necessary. When the compound forming the gate insulating film has a wider Eg than the GaN and the ΔEc and the ΔEv are present, breakdown voltage is high and film characteristics are favorable.

FIG. 3 shows the band structure of each compound. First, GaN is represented by Eg=3.3 ev, based on a CB (conduction band) and a VB (valence band). Regarding SiO₂, Eg=9.0 ev can be obtained by adding the values of an ΔEv and an ΔEc to 3.3 ev. Similarly, the Eg of each of HfO₂ and ZrO₂ is 6.0 ev; the Eg of Al₂O₃ is 8.8 ev; the Eg of La₂O₃ is 6.0 ev; and the Eg of Y₂O₃ is 5.6 ev. Accordingly, it is apparent that the Eg of SiO₂ is the largest.

In other words, in the semiconductor transistor 11, the first insulating film 6 was formed with a compound having a higher oxidization power than SiO₂ to provide favorable interface characteristics between the gate insulating film and the active layer 3, and the second insulating film 7 was formed with a compound having a wider band gap than the first insulating film 6 to provide favorable film characteristics of the gate insulating film.

Hereinafter, a method for producing the semiconductor transistor 11 is described. First, as shown in FIG. 4A, the buffer layer 2 composed of AlN or GaN and having a thickness of approximately 20 nm and the p-GaN layer 3 having a thickness of approximately 1 μm are sequentially grown on the substrate 1 made of, for example, sapphire, SiC, Si, GaN or the like by a metal organic chemical vapor deposition (MOCVD) method. For example, Mg is used as a p-type dopant, and the dopant concentration is, for example, set to 1×10¹⁶ to 1×10¹⁷/cm³. The growth of GaN or the like on the substrate 1 is not limited to the MOCVD method, and other growth methods such as a hydride vapor phase epitaxy (HVPE) method or a molecular beam epitaxy (MBE) method may be used.

Next, as shown in FIG. 4B, a photoresist 4 is applied to the p-GaN layer 3 and exposed to light and developed to form openings in a source region and a drain region. After that, through the openings, an n-type dopant such as silicon is injected to form the n+ type source region 5 s and the n+ type drain region 5 d. In this case, the concentration of the n-type dopant is set to, for example, 1×10¹⁸ to 2×10²⁰/cm³.

After the photoresist 4 is removed with a solvent, as shown in FIG. 4C, an alumina film (Al₂O₃) that is to be the first insulating film 6 is grown so as to have a thickness of, for example, from 5 nm to 10 nm, by a plasma-free catalytic chemical vapor deposition (Cat-CVD) method. The Cat-CVD method is performed, for example, using the film formation apparatus 21 shown in FIG. 5.

The film formation apparatus 21 includes a chamber 22, as shown in FIG. 5. In the chamber 22 an exhaust outlet 23 and a substrate-conveying opening 24 are provided. Additionally, inside the chamber 22 a substrate-mounting table 25 for mounting the substrate 1, a gas-discharging device 26, and a linear catalyst 27 are disposed. The gas-discharging device 26 discharges, to the substrate-mounting table 25, a reaction (raw material) gas which has been introduced from outside. The catalyst 27 is composed of tungsten and is placed between the substrate-mounting table 25 and the gas-discharging device 26.

Here, the substrate 1 on which the p-GaN layer 3 has been formed is mounted on the substrate-mounting table 25, the pressure inside the chamber 22 is reduced, and an electric current is supplied from an alternating current source (not shown) to the catalyst 27 to achieve a state in which the temperature thereof is increased to approximately from 1800° C. to 1900° C. In this state, a raw material gas is discharged from the gas-discharging device 26 to the p-GaN layer 3. The raw material gas is decomposed by the catalyzer 27 to produce an active species, whereby the alumina film (first insulating film) 6 is formed on a surface of the p-GaN layer 3.

Subsequently, as shown in FIG. 4D, a silicon oxide film (SiO₂ film) that is to be the second insulating film 7 is formed, on the alumina film 6, so as to have a thickness of approximately from 50 nm to 100 nm.

Next, as shown in FIG. 6A, a conductive film 8 is formed on the silicon oxide film 7. Polysilicon is generally used for the conductive film 8, but the conductive film 8 is not restricted thereto, and may be a metal film such as Ni/Al or WSi. In the case of using polysilicon, As, P (phosphorous), B (boron), or the like is doped and polysilicon is grown by the CVD method, whereas, in the case of a metal film, it is formed by sputtering or the like.

Furthermore, a photoresist 9 is applied to the conductive film 8, and is exposed to light and developed, as a result of which, the photoresist 9 remains in the gate region, but is removed from the source region 5 s and the drain region 5 d.

Then, as shown in FIG. 6B, using the patterned photoresist 9 as a mask, etching is performed on the conductive film 8, the silicon oxide film 7, and the alumina film 6, and the conductive film 8 left on the gate region serves as the gate electrode 8 g. The silicon oxide film 7 and the alumina film 6 under the gate electrode 8 g serve as the gate insulating film.

Next, as shown in FIG. 6C, the photoresist 9 is removed, and then, as shown in FIG. 1, a source electrode 10 s is formed on the source region 5 s and simultaneously, a drain electrode 10 d is formed on the drain region 5 d, by a lift-off method using another photoresist (not shown). Each of the source electrode 10 s and the drain electrode 10 d is a film composed of, for example, Ti/Al, Ti/AlSi, Mo, or the like and is in ohmic contact with the n+-GaN layer that forms the drain region 5 d and the source region 5 s. Through the above processes, the semiconductor transistor 11 shown in FIG. 1 can be produced.

In the semiconductor transistor 11 of the embodiment, the gate insulating film was formed as a hybrid film composed of the first insulating film 6 and the second insulating film 7, in which the first insulating film 6 was made of one or more compounds selected from the group consisting of Al₂O₃, HfO₂, ZrO₂, La₂O₃, and Y₂O₃ and having a higher oxidization power than that of SiO₂, and the second insulating film 7 was made of SiO₂ as a compound having a wider band gap than the first insulating film 6.

Accordingly, in the semiconductor transistor 11, the interface characteristics between the GaN-based semiconductor and the gate insulating film are favorable due to the first insulating film 6, and a high breakdown voltage is obtained and the film characteristics of the gate insulating film are favorable due to the second insulating film 7. Therefore, the semiconductor transistor 11 has both favorable interface characteristics between the GaN-based semiconductor and the gate insulating film and favorable film characteristics of the gate insulating film.

It is envisaged that the semiconductor transistor 11 will be used, for example, as a power device for automobiles, home appliance power circuits, and the like, but not in an LSI of a so-called signal system. Thus, since the film thickness is increased as a result of using the hybrid film as the gate insulating film, the semiconductor transistor 11 can be suitably used as a power device.

Hereinabove, the present invention has been described based on a preferable embodiment. However, the semiconductor transistor according to the present invention is not restricted simply to the configuration of this embodiment. Various changes and modifications in the configuration of the embodiment are also encompassed in the scope of the invention.

The entire disclosure of Japanese Patent Application No. 2010-045547 is incorporated herein by reference.

All publications, patent applications, and technical standards described in the present specification are incorporated herein by reference to the same extent as if each publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

DESCRIPTION OF REFERENCE NUMERALS

1: Substrate

2: Buffer layer

3: Active layer

4, 9: Photoresist

5 s: Source region

5 d: Drain region

6: Alumina film (first insulating film)

7: Silicon oxide film (second insulating film)

8: Conductive film

8 g: Gate electrode

10 s: Source electrode

10 d: Drain electrode

11: Semiconductor transistor 

1. A semiconductor transistor comprising: an active layer composed of a GaN-based semiconductor; and a gate insulating film having a first insulating film including one or more compounds selected from the group consisting of insulating films in which an absolute value of standard enthalpy of formation is larger than 228 kJ/mol, and a second insulating film composed of SiO₂ formed on the first insulating film.
 2. A semiconductor transistor comprising: an active layer made of a GaN-based semiconductor; and a gate insulating film having a first insulating film including one or more compounds selected from the group consisting of Al₂O₃, HfO₂, ZrO₂, La₂O₃, and Y₂O₃, formed on the active layer, and a second insulating film composed of SiO₂ formed on the first insulating film. 